From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F01CDC04A6B for ; Fri, 10 May 2019 12:04:41 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C4B7520989 for ; Fri, 10 May 2019 12:04:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C4B7520989 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([127.0.0.1]:42347 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hP4GW-000379-UK for qemu-devel@archiver.kernel.org; Fri, 10 May 2019 08:04:40 -0400 Received: from eggs.gnu.org ([209.51.188.92]:44038) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hP4FZ-0002oF-JT for qemu-devel@nongnu.org; Fri, 10 May 2019 08:03:42 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hP4FX-0006KW-IW for qemu-devel@nongnu.org; Fri, 10 May 2019 08:03:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38254) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hP4FX-0006IE-BQ for qemu-devel@nongnu.org; Fri, 10 May 2019 08:03:39 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 169F03082137; Fri, 10 May 2019 12:03:36 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-116-28.ams2.redhat.com [10.36.116.28]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C2AF55D9D5; Fri, 10 May 2019 12:03:34 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 4972E1132B35; Fri, 10 May 2019 14:03:33 +0200 (CEST) From: Markus Armbruster To: Kashyap Chamarthy References: <20190510081526.15507-1-kchamart@redhat.com> Date: Fri, 10 May 2019 14:03:33 +0200 In-Reply-To: <20190510081526.15507-1-kchamart@redhat.com> (Kashyap Chamarthy's message of "Fri, 10 May 2019 10:15:25 +0200") Message-ID: <874l62sei2.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.42]); Fri, 10 May 2019 12:03:36 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH v2] VirtIO-RNG: Update default entropy source to `/dev/urandom` X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: rjones@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com, amit@kernel.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kashyap Chamarthy writes: > When QEMU exposes a VirtIO-RNG device to the guest, that device needs a > source of entropy, and that source needs to be "non-blocking", like > `/dev/urandom`. However, currently QEMU defaults to the problematic > `/dev/random`, which is "blocking" (as in, it waits until sufficient > entropy is available). > > Why prefer `/dev/urandom` over `/dev/random`? > --------------------------------------------- > > The man pages of urandom(4) and random(4) state: > "The /dev/random device is a legacy interface which dates back to a > time where the cryptographic primitives used in the implementation > of /dev/urandom were not widely trusted. It will return random > bytes only within the estimated number of bits of fresh noise in the > entropy pool, blocking if necessary. /dev/random is suitable for > applications that need high quality randomness, and can afford > indeterminate delays." > > Further, the "Usage" section of the said man pages state: > > "The /dev/random interface is considered a legacy interface, and > /dev/urandom is preferred and sufficient in all use cases, with the > exception of applications which require randomness during early boot > time; for these applications, getrandom(2) must be used instead, > because it will block until the entropy pool is initialized. > > "If a seed file is saved across reboots as recommended below (all > major Linux distributions have done this since 2000 at least), the > output is cryptographically secure against attackers without local > root access as soon as it is reloaded in the boot sequence, and > perfectly adequate for network encryption session keys. Since reads > from /dev/random may block, users will usually want to open it in > nonblocking mode (or perform a read with timeout), and provide some > sort of user notification if the desired entropy is not immediately > available." > > And refer to random(7) for a comparison of `/dev/random` and > `/dev/urandom`. This is Linux. What about other supported POSIX[*] hosts? If any such host has /dev/random that works here, but not /dev/urandom, we regress. *If* there's an actual regression risk: a simple & stupid way to reduce it risk could be falling back to /dev/random when opening /dev/urandom fails. Perhaps only when it fails with ENOENT. Possible implementation: instead of setting a default filename in rng_random_init(), change rng_random_opened() to try /dev/urandom, then /dev/random when filename is still null. Aside: "opened" sounds like a predicate. Goes back to commit a9b7b2ad7b0. > Given the above, change the entropy source for VirtIO-RNG device to > `/dev/urandom`. > > Related discussion in these[1][2] past threads. > > [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html > -- "RNG: Any reason QEMU doesn't default to `/dev/urandom`?" > [2] https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html > -- "[RFC] Virtio RNG: Consider changing the default entropy source to > /dev/urandom" > > Signed-off-by: Kashyap Chamarthy [*] POSIX because common-obj-$(CONFIG_POSIX) += rng-random.o